The present invention relates in general to method and apparatus for controlling the depth of marine cables, and in particular to such depth controllers which are remotely controlled electronically from the ship which tows the cable.
Remote controlled seismic cable depth controllers have in the past been of two general types or categories.
A first type, such as in U.S. Pat. Nos. 3,412,704; 3,541,989; and 3,605,674, has been based on controlling the depth of depth controllers or paravanes disposed at spaced positions on the cables by means of signals sent from transmitters mounted in the seismic cable near each controller to receivers in the controllers. Components of the controller adjusted the depth of the cable in response to the signals received at the receiver. These types of apparatus thus required a special cable for remote control seismic surveying, containing transmitters at spaced locations therein. Since typical seismic cables for survey operations may reach one or two miles in length, it was undesirable for economic reasons to require a special cable, different from that normally used, for remote control seismic surveying.
A second type of cable depth controller has used signal responsive sensors in the depth controller to receive signals sent through a conductor in the cable (as in U.S. Pat. Nos. 3,648,642 and 3,673,556) or through the water (as in U.S. Pat. No. 3,412,704). However, the number of depths to which the controllers could be raised or lowered oftentimes depended on the number of frequencies used. As the number of frequencies increased, the number of frequency-sensitive filters or frequency-sensitive relays in the controllers increased. The sensitivity of the frequency-sensitive members had to increase, so far as is known, if several frequencies were attempted for several different depths. Also, since analog signals often were used with cycles of frequency specifying the desired depth, and these frequencies could drift, this type of depth controller could deviate from the desired depth setting and affect cable depth.
Another type of cable depth controller employing a conductor in the cable has featured a digital control signal sent down the conductor. The digital control signal representing the desired operating depth of the cable was received by the depth controller and stored in a shift register. The depth control signal was compared with an encoded signal representing the present setting of a motor which controlled the compressive force exerted by a spring on a piston exposed to ambient water pressure at the actual operating depth of the seismic cable. Differences between the water pressure and spring pressure causes adjustment of the angle of the diving planes on the depth controller. This type of system is set forth in U.S. Pat. No. 4,290,124, assigned to the assignee of the present invention. While this cable depth controller has provided superior results, it lacked the capability of transmitting data back to the ship. Also, while it used a feedback control loop for maintaining depth, it lacked optimum responsiveness in maintaining the desired depth in adverse conditions.